1. Field of the Invention
The present invention relates generally to monitoring of coatings applied to webs or sheets. More particularly, this invention relates to a method of monitoring the drying speed of these coatings.
2. Description of the Prior Art
Application of coatings to sheets or webs of material is a common process in a number of different industries. Particularly in the paper industry, coatings, containing components such as clay, latex, or CaCO.sub.3, are used to affect the absorption, stability, gloss, printability or other characteristics of the underlying web or sheet. In other industries, such as plastic, the coating may be the most important part of the product (i.e. photographic coating).
In an on-line setting, coatings are typically applied using numerous types of coating devices (i.e. coaters). Typically, these devices hold the coating in liquid form, suspended in a solvent, such as water, to ease application and improve bonding to the base sheet. A blade or knife is used to spread the coating along the base sheet as it moves past the coating device. The coating device is followed by one or more drying devices (i.e. dryers) which evaporate the solvent out of the coating, using heat and/or moving air, leaving a sold or semisolid coating layer behind on the base sheet.
The speed at which the coating dries can be a critical factor in coating quality and the quality of the final sheet product, as well as a factor in mill or plant efficiency, and the amount energy used during the coating application process. Factors which effect coating drying speed include the mill environment, coater settings, dryer settings, base sheet properties, as well as the quantity and type of components in the coating. Further, the type and amount of solvent--which evaporates and/or is absorbed by the base sheet upon drying of the coating--also effects the drying time of the coating.
As the reader is likely aware, the quality of applied coatings is important for a number of reasons. Visible variations in quality and uniformity of a coated sheet are clearly something which should be avoided. Further, non-visible variations can be critical where an applied coating is to be used as the base for further processing, such as printing. Lastly, a coating may provide mechanical durability and protection against environmental factors such as moisture, heat or sunlight.
The speed at which a coating dries effects coating quality and final sheet quality in a number of ways. When the sheet is wet, the heat in the dryers is absorbed by the coating as the coating solvent proceeds through a state change from a liquid to a vapor. Once the coating or coating surface is dry (i.e. most of the coating solvent has evaporated) however, the excess heat is no longer absorbed by the coating solvent. Rather, it is absorbed by the base material and coating, which may burn or crack easily in their dry state. If, on the other hand, the coating dries too slowly, the coating may be smeared or stick to manufacturing line components, or other sections of the sheet (i.e. if the sheet is rolled while the coating is still wet). Thus, monitoring drying speed can be used to improve overall coating and final product quality, as well as prevent product waste.
Monitoring coating drying speed has another advantage as well. It can reduce the amount of wasted energy and increase dryer lifetime by allowing unneeded drying devices to be turned off or selected drying devices to be set at lower temperature settings, when the coating dries prior to the end of a section of coating dryers. Turning off unneeded dryers in fact, is one method of preventing the sheet from being burned or otherwise damaged due to the excessive heat in the dryers.
Currently, temperature sensors are used to monitor when a coating is dry in many sheet mills and plants. Typically, the temperature sensors are located in or near the dryers following the coater, and register an increase in sheet temperature when the coating is dry. Due to the harsh temperature in these drying environments, thermocouples or other rugged temperature devices are preferred to measure temperature within the dryers. Unfortunately, thermocouples are a relatively slow temperature-measuring device compared with the speed at which the sheet is moving past the dryers. Using temperature to monitor the drying speed of the coatings is also disadvantageous because once the temperature sensors register the increase in sheet temperature, damage to the sheet may have already occurred due to over-drying.
While remotely-located IR temperature sensors have also been used, these types of sensors are not very accurate, and their readings depend on surface emmisivity, which changes during drying.
Ideally, monitoring of drying speed would be accomplished by monitoring the location of one or more "gel points" of the coating. The gel point defines the time at which particular coating components form semisolid networks of solid aggregates, as the coating solvent evaporates from the applied coating layer. When most or all the primary coating components have reached this gel point, the coating is effectively considered to be dry.
Currently, no device exists to monitor the gel point of coatings on sheets or webs of material.